Research suggests that timing of physical maturation in girls serves as a pivotal juncture for future emotional stability. Compared to later maturing peers, girls who experience an earlier onset of puberty are at increased risk for a variety of troubling outcomes. This association is typically interpreted causally: early puberty initiates a cascade of psychosocial consequences that produce pathological outcomes. This explanation, however, neglects the importance of genetic predispositions both for pubertal timing and for correlated psychopathology. The proposed project utilizes behavior genetics methodology and data from the Australian Twin Registry to advance understanding of the effects of early physical maturation on the emergence of adolescent problem outcomes. Analyses extend beyond simple heritability estimates; rather, the project aims to assess the causal relation between menarche on adolescent symptomatology and to provide a basis for evaluating pre-existing theories of early menarche. Though distinctive in its synthesis of complex methodology and theoretical questions, this project also has direct clinical relevance. A broader understanding of the mechanism by which normal developmental processes go awry will facilitate treatment.